A conventional synchronous terminal receiving station system consists of a large casing frame that is installed on the floor. In side this casing is housed at least one main receiving unit and one standby receiving unit. The main receiving unit receives and demultiplexes high-speed higher order group signals in which a plurality of frame-synchronized low order group signals are multiplexed, through a transmission path. The synchronous terminal station receiving system also has a monitor circuit to monitor the main transmission unit. When the unit fails the main transmission path is switched to the standby transmission path. Such a communication system is disclosed in U.S. Pat. No. 4,601,028.
However, in cases where a synchronous terminal station system has to receive and demultiplex transmission path signals multiplexed a larger number of low order group signals, it becomes impossible to integrate all of the main receiving units and a standby receiving unit into one casing frame because in this case the scale of the circuits becomes larger. For this reason, it is necessary to provide separate casing frames for each main receiving unit and the standby receiving unit.
An explanation on work of a such conventional main receiving unit is performed as below. In this case, after being demultiplexed, signals inputted from a main transmission path are moved in a pointer conversion circuit section on a clock and a local frame in a casing frame in which the main receiving unit is housed. Also, the main receiving unit modifies the pointer value of the signals corresponding to its pointer value and transmits them to a low order group transmission path. However, when the main transmission path or the main receiving unit fails, the transmission path is switched to a standby transmission path. After being demultiplexed in the standby receiving unit, the signals inputted from the standby transmission path are moved in the pointer conversion circuit section of the standby receiving unit on a clock and a frame within the unit in which the standby receiving unit is housed. Also, the standby receiving unit modifies the pointer value of the signals corresponding to its pointer value and transmits them to the frame in which the main receiving unit is housed. The low order group signals inputted from the standby unit is transmitted to a low order group transmission path. However, as the main receiving unit and the standby receiving unit are housed in frames separately, the clocks in each frame are independent of each other. Therefore, when the low order group signals inputted from the standby receiving unit is switched to be transmitted to the low order group transmission path in an abnormal condition, the clock phase and the frame phase of the low order group signals transmitted to the low order group transmission path are modified. For this reason, in the main receiving unit the clock phase and the frame phase have to be matched to the clock and the frame within the unit in the frame casing in which the main receiving unit is housed in the both cases of transmitting the low order group signals demultiplexed in the main receiving unit to the low order group transmission path and transmitting the low order group signals demultiplexed in the standby receiving unit to the low order group transmission path. To accomplish this, the low order group signals have to be stored in a buffer memory for the time required to adjust the phases. In this case, each buffer memory for each low order group signal must have a memory capacity for one frame of the low order group signals. For example, when STM-1 (STM:Synchronous Transport Module) signals of CCITT Recommendation G.708 are given as the low order group signals, high-speed memories of a large capacity equivalent to 19440 bits for a frame of a STM-1 signal (155,52 Mb/s) are required. This results in the scale of the circuitry becoming extremely large.